List extending Collection/SequencedCollection

[Ryan Ernst]

Hi core-libs-dev,

I know various threads have existed over the past few months on
SequencedCollection and its implications on compatibility. I wanted to
raise this semi-related issue we noticed recently after beginning
testing against Java 21.

Elasticsearch began testing against Java 21, and noticed a series of
failures in Painless (our builtin scripting language, which
dynamically compiles to bytecode). Most tests that used List failed to
compile, with several unknown methods (eg add). Painless builds a
hierarchy of classes it knows about for method resolution. This
hierarchy didn't know anything about SequencedCollection since we
currently compile against Java 17. Now, this was ultimately a bug in
Painless, because we knew there could be cases where we encounter
unknown classes in the hierarchy (eg a class which is not blessed to
be visible in the language). However, I think the reason we found that
bug (List extending from SequencedCollection) might still cause issues
for some.

While debugging the issue, something that struck me as interesting in
the SequencedCollection hierarchy is the difference between List and
SortedSet. Both of these classes were made to be compatible with
sequenced classes by adding the new classes as super interfaces,
SequencedCollection and SequencedSet, respectively. However, while
SortedSet was *added* as a super interface, List had its super
interface Collection *replaced* by SequencedCollection.

I don't know enough about the rampdown process to know if this is
still fixable in Java 21. It is probably an extreme edge case that
doesn't matter, since eg instanceof Collection will still work in
existing code. But for consistency, it would seem better to maintain
Collection as a direct super interface of List. Is there any reason
such a change doesn't fit with the collections architecture going
forward?